Secure Communication And Resource Allocation Schemes Based On Relay And Reconfigurable Intelligent Surface

The next generation mobile communication has higher requirements for performance indicators including energy efficiency,data rate and coverage.It means that the contradiction between limited resources and the increasing communication service demands needs to be solved urgently.Cooperative relay technology has become one of the most potential solutions to solve the above problem.However,the relay participating in cooperation may be untrusted.The relay assists in forwarding the message,while the message is confidential to the relay.Therefore,it is urgent to solve the contradiction between the openness of wireless network and the security of information system.Reconfigurable intelligent surface(RIS)has attracted extensive attention since it can control the wireless propagation environment and enhance the security of communication system.Based on the above research background and development trend,novel,efficient and secure transmission schemes in wireless networks need to be explored in the future.In this dissertation,the physical layer security of untrusted relay system is regarded as the core,and the secrecy rate optimization problem of the wireless relay communication system was investigated.To solve the problems of energy loss and weak security in the untrusted multirelay system,the secrecy rate optimization based on optimal relay selection was investigated by introducing the power splitting and the time switching protocols.If a power splitting protocol is adopted,then the relay will work in the half-duplex mode.While if the time switching protocol is employed,then the relay will work in the fullduplex mode to enhance transmission efficiency.Based on the power splitting and the time switching protocols,the expressions of the secrecy rate under imperfect interference cancellation were derived respectively.Under the total power constraint,the secrecy rate was maximized by the optimal power factor and the time factor obtained with the interior point method.In addition,the optimal relay was selected to further maximize the secrecy rate.It is experimentally verified that the secrecy rate of the proposed scheme is significantly improved compared with the benchmark schemes.The existing RIS assisted relay systems only consider the situations of trusted relay and external eavesdropper.The secrecy rate optimization of RIS assisted untrusted relay system was studied.The base station sends both confidential signals and jamming ones to prevent the untrusted relay from eavesdropping.The RIS reflects the confidential signal in the same phase to enhance the received signal strength of legitimate users,and reflects the jamming signal to the untrusted relay in an inverted manner.To further improve the secrecy rate,the RIS phase shift,the power allocation between confidential signal and jamming signal,and that between base station and untrusted relay were optimized jointly under the constraints of unit modulus and total power.The complex multivariable coupling problem was decomposed into three sub-problems.The RIS phase shift was resolved with semidefinite relaxation method,and the power allocation was resolved with successive convex approximation method.It is indicated that the secrecy rate of the proposed scheme is remarkably enhanced compared with the three benchmark schemes.To solve the problems of small coverage and low rate in the relay and RIS assisted non-orthogonal multiple access system,a joint optimization scheme for phase shift and power allocation was proposed.In addition,two practical communication scenarios of the trusted and untrusted relays were considered,respectively.When the relay is trusted,under the constraints of unit modulus and total power,the RIS phase shift and the power allocation were jointly optimized to maximize the secrecy rate.When user 1 and the relay are at different security levels,the secrecy rate of user 1was maximized by optimizing the RIS phase shift and the power allocation under the three constraints of the unit modulus limit at the RIS,the total power limit and quality of service limit of user 2.The non-convex problem was resolved by using the semidefinite relaxation method,the Taylor formula and the Gaussian randomization.Simulation results show that the achievable sum rate and the secrecy rate of the system can be improved by jointly designing RIS phase shift and total power allocation.